This invention relates generally to digital modulator-demodulators (modems), and, more particularly, to techniques for the synchronization of modulators and demodulators. In communications systems, digital information is typically transmitted by encoding it onto a high-frequency oscillatory signal known as a carrier signal. The carrier signal is said to be modulated by the information signals, and in general the modulation of the carrier signal may be to its amplitude, frequency or phase, or a combination of these.
In the transmission of digital data, the modulation technique often employed is phase shift keying, in which the phase of the carrier signal is changed or shifted by a preselected amount to indicate a discrete informational element known as a baud. The baud may represent one or more binary digits or bits of information. Although phase shift keying is mentioned from time to time in this specification, it will be understood that the invention to be described is not limited to a particular modulation approach.
In digital modems, not only is the data to be transmitted in digital form but the carrier signal is manipulated in the modems in digital form. At the sending end of the communication system, digital samples representing the modulated carrier signal are converted to an analog signal for transmission, and, at the receiving end of the system, the received analog signals are first digitized in an analog-to-digital converter.
From this brief introduction, one can appreciate that it is highly desirable to synchronize the demodulation and modulation equipment as closely as possible. However, since the demodulator and modulator are usually located remotely from each other, this is not always a simple matter. Prior to this invention, the technique invariably employed was to provide a variable-frequency oscillator to control the sampling rate at the receiving end, and to synchronize this sampling rate with the modulation rate, or baud rate, by means of a conventional phase-locked loop.
Although synchronization by means of a phase-locked loop is satisfactory in many applications, it is most unsatisfactory in any system in which the modem function is performed by a computer or processor that is also used either to perform other computational functions, or to perform the functions of multiple modems. Whenever a conventional serial processor is used for more than one function, priorities must be assigned such that the most important functions are performed first. If synchronization is to be accurately maintained, then clearly the modem function would have to have the highest priority. But if the same processor is handling multiple modems, the accuracy of synchronization will necessarily suffer to some degree. In many applications, the modem processor is also emoloyed to process data in "real time," i.e., as soon as possible after the occurrence of some real event from which the data being processed is derived. An example is the processing of speech in digital form. Such real-time processing frequently also requires the highest level of priority if the integrity of the data is to be maintained. Obviously, then, performance will be degraded in some manner in such a situation, either because synchronization has not been accurately maintained, or because the processing of the data has not been given the priority it demands.
It will be apparent from the foregoing that there is a significant need in the modem field for a synchronization technique that avoids these problems arising from the use of a variable-frequency oscillator in conjunction with a phase-locked loop. The present invention fulfills this need.